People smugglers and criminal gangs in Libya are using social media to broadcast the abuse and violence they inflict on African migrants in their captivity and demand ransoms from their families back home, according to the United Nations.

In a video posted on Facebook, hundreds of emaciated Somalis and Ethiopians, including several children, are seen huddled in a concrete room in an unknown location in Libya.

The migrants and refugees being filmed say they have been beaten, tortured and held in cells without food, and that their parents and relatives have received video clips via social media asking for up to $10,000 to spare them from being killed.

“They broke my teeth … they broke my hand … this stone has been put on me for the last three days,” says one man in the video posted last week, explaining how his captors placed a concrete block on his back as a punishment after his family refused to pay up $8,000.

At least 20,000 migrants are being detained in Libya, the main gateway for those attempting to reach Europe by sea, according to the International Organization for Migration (IOM).

Many are held and extorted for money by smugglers and gangs, and rising numbers are traded – in what they call slave markets – for forced labour and sexual exploitation, the U.N. agency says.

“The IOM condemns the way that criminal gangs use social media in their shocking abuse of people held against their will and to extort money from their families back home,” said Mohammed Abdiker, IOM director of operations and emergencies.

“This is a global problem where a smuggler or a criminal gang can easily use digital platforms to advertise their services, entice vulnerable people on the move and then exploit them and their families,” Abdiker said in a statement.

Several Senegalese migrants who were flown home by the IOM from Libya last week told the Thomson Reuters Foundation of the ‘hell’ they endured in detention, ranging from being beaten and starved to watching their peers die of hunger and illness.

The voyage from Libya across the Mediterranean to Italy – often on flimsy boats run by people smugglers – has become the main route to Europe for migrants from Africa after a European Union crackdown last year on sea crossings from Turkey.

Smugglers in increasingly lawless Libya are packing record numbers of migrants onto boats, with sea arrivals to Italy so far this year – more than 61,000 people – up 35 percent on 2016.